Document the default Net timeout values

Currently there is no documentation of the default timeout values in the Net libraries.

For example, Net:HTTP has a default [open_timeout](http://ruby-doc.org/stdlib-2.0/libdoc/net/http/rdoc/Net/HTTP.html#open_timeout-attribute-method) of nil (i.e. never timeout) and a [read_timeout](http://ruby-doc.org/stdlib-2.0/libdoc/net/http/rdoc/Net/HTTP.html#read_timeout-attribute-method) of 60 seconds.

This patch adds the documentation for the defaults of all the timeout options in Net::FTP, Net:HTTP, Net::POP and Net::SMTP.

If this patch is accepted I can backport it to the Ruby 1.9 branch.